#433 Add support for pure Wayland system

[lostdistance]

pcmanfm has an unconditional dependency on libX11, within the source file src/desktop.c and within the configuration files.

This precludes building and running pcmanfm on a pure Wayland system, i.e. one without libX11.

I do not know whether the existing libX11 function calls can be replaced by equivalent GTK (or other) function calls. If they can would you please consider removing the dependency on libX11, or at least making it conditional.